1. Field of the Invention
The present invention concerns a method and a device for manufacturing an insulative material cellular insulator around a conductor. It applies more particularly, but not in any limiting way, to the manufacture of dielectric intermediate insulators of coaxial cables.
2. Description of the Prior Art
Coaxial cables usually include a solid or stranded central conductor surrounded by an intermediate insulator made of an insulative dielectric material in turn surrounded by an outer conductor protected by a protective outer sheath. The dielectric intermediate insulator must have specific dielectric properties in order to obtain the attenuation characteristics required of the cable, in particular at high frequencies. To be more precise, this insulator is generally required to have a dielectric constant less than about 1.8 and as close as possible to 1. The closer the dielectric constant to 1, the higher the frequencies at which the cable can be used.
The insulative materials conventionally used in cablemaking do not have dielectric constants of this order when they are used in solid form. Their dielectric constants are generally close to 2. This applies to polyethylene and to polytetrafluoroethylene (PTFE) in particular. The use of these materials to form cellular insulators to reduce the dielectric constant is known in itself.
Cellular insulators are those into which a plurality of bubbles filled with air or with a gas having a dielectric constant close to 1 are introduced during the application (generally by extrusion) of the insulative material to form an insulator, and usually by means of a chemical reaction. The present invention is not concerned with this type of insulator.
Cellular insulators, with which the present invention is concerned, have cells extending longitudinally (in a straight line or in a helix) along the cable and separated from each other by radial walls, the cells being obtained by shaping the insulative material used, which in this case is solid, using an extrusion device having the appropriate openings and passages for this purpose. The cells are entirely closed, with the result that the intermediate insulator is cylindrical or polygonal and its cross-section resembles a spoked wheel.
One method of manufacturing a cylindrical cellular intermediate insulator in which the cells are entirely closed is described in U.S. Pat. No. 3,771,934. It involves extruding the insulative material, in the viscous state of course, imparting the required shape to it by shaping means to form the cells, applying the material formed in this way to the central conductor, and finally cooling this insulative material to obtain the insulator.
In this method, the material shaped by extrusion is applied to the central conductor immediately on leaving the shaping means. Accordingly, to prevent thinning of the very thin top walls of the cells (those adapted to come into contact with the outer conductor of the cable) it is necessary to pressurize the cells during manufacture. This complicates manufacture.
Moreover, the shaped material passes suddenly from the guide to the conductor, which causes it to undergo a large variation in diameter which can cause longitudinal cracks in the shaped insulator.
Finally, this method cannot be used to manufacture coaxial cables with cellular insulation having a low dielectric constant and a small diameter intermediate insulator.
A first aim of the present invention is therefore to develop a method of manufacturing a cellular insulator around a conductor which enables pressurization of the cells to be dispensed with.
Another aim of the present invention is to develop a method of this kind that is free of the risk of cracking of the shaped insulator.